The present invention relates generally to a method of purging materials (interfering substances) from a chamber, and, more particularly, to a method of purging the interfering substances from an atmosphere around an optical system using an inert gas in an exposure apparatus for exposing a substrate, such as a single crystal substrate for a semiconductor device and a glass substrate for a liquid crystal display device (“LCD”). The present invention also relates to an exposure apparatus to which the purge method is applied, and a device manufacturing method that uses the exposure apparatus.
A reduction projection exposure apparatus has been conventionally employed which uses a projection optical system to project a circuit pattern formed on a mask (or a reticle) onto a wafer, etc., to transfer the circuit pattern, in manufacturing such a fine semiconductor device as a semiconductor memory and a logic circuit in photolithography technology. Recently, semiconductor integrated circuits have increasingly improved the degree of integration, and the present trend requires a nano order of fine processing. For this purpose, the exposure apparatus is using a laser with a short wavelength for an exposure light source. In addition, in order to maintain stable exposure performance, the exposure apparatus is designed to maintain high cleanability, and be used in an atmosphere that contains extremely little in impurities, such as a clean room.
However, as the exposure light source laser has a shorter wavelength, the exposure light causes the residual material in the exposure apparatus to photochemically react with oxygen (O2), and reaction products, such as (NH2) SO4, to adhere to and blur optical elements, such as a lens and a mirror, disadvantageously deteriorating the light intensity.
Therefore, the exposure apparatus, which uses a KrF excimer laser and an ArF excimer laser for a light source, adopts a method for purging the atmosphere around the optical elements arranged on the laser's optical path with the inert gas in order to prevent not only a deteriorated light intensity due to the impurities' adhesions to the optical elements, but also reduced transmittance due to the light absorption in oxygen, etc., in the atmosphere on the optical path.
For example, an exposure apparatus 500, which has an internal structure schematically shown in FIG. 11, generally includes a laser light source 100, an illumination optical system 102 for converting into light having a predetermined shape, a laser beam 100a that serves as the illumination light emitted from the laser light source 100, and a projection optical system 103 that images onto the wafer (substrate) 100W, the shaped laser beam 100a that has passed a reticle (or an original form) 100R.
The illumination optical system 102 includes plural optical elements, such as plural types of lens units 104a, 104b and 104c and mirrors 105a and 105b, and serves to irradiate the laser beam 100a onto an illumination area on the reticle 100R. The illumination optical system 102 and the projection optical system 103 are sealed in chambers 102a and 103a, respectively, to which a nitrogen supply unit 106 for supplying nitrogen gas (or inert gas) 106a is connected via nitrogen gas supply lines 109a and 109b and the mass flow controller 110a and 110b. 
Gas exhaust lines 111a and 111b are connected to an exhaust port via valves 112a and 112b. In purging, with the nitrogen gas, the inside of the chamber 102a of the illumination optical system and the inside of the chamber 103a of the projection optical system, these gas exhaust lines 111a and 111b exhaust inside impurities and always maintain the insides of the chambers 102a and 103a to be clean. The “impurities”, as used herein, means the residual materials in the chamber other than nitrogen gas as the purge gas, and cover oxygen, organic materials, reaction products between oxygen and the organic materials, outgas, water, etc. The outgas is emitted gas, which is gradually emitted from surfaces of respective components, such as an optical element and a barrel, housed in the chamber.
When the facilities of the exposure apparatus 500 stop, the valves 112a and 112b close so as to protect the insides of the chambers 102a and 103a from the ambient atmosphere. The chamber 102a is provided with oxygen concentration detectors 113a, 113b and 113c, and the chamber 103a is provided with an oxygen concentration detector 113d. Prior to exposure, these detectors detect the oxygen concentrations in the chambers 102a and 103a, so as to confirm whether the passage atmosphere of the laser beam 100a is sufficiently replaced with nitrogen gas and whether the exposure performance can be maintained.
The illumination optical system 102 needs exchanges of components when the performance of the optical element deteriorates due to contaminations caused by the photochemical reactions and long-term use. In addition, in order to adjust an output of the laser beam 100a (exposure light) in accordance with the exposure plan, a user may change a dimmer filter (not shown) provided to the illumination optical system 102 to another type. Therefore, the inside of the chamber 102a in the illumination optical system is often released to the air and the impurity concentration becomes frequently high.
As one maintenance called optical cleaning, the exposure apparatus 500 generates ozone by introducing oxygen into a space on the exposure optical path, and uses ozone to remove contaminants adhered on the optical elements. Even after this maintenance, the oxygen concentration sometimes becomes high in each of the chambers 102a and 103a for respective optical systems and misses the exposure condition. In this case, the exposure apparatus should inefficiently wait for the exposure for a certain time period of the nitrogen gas purge (while this standby time is called downtime) until the oxygen concentration reduces down to an exposable state.
For an effectively shortened downtime, one proposal has been already made (see, for example, Japanese Patent Application, Publication No. 6-216000). This proposal increases and decreases the flow rate of the purge gas based on the oxygen concentration in the exposure apparatus, and promptly replaces the inside of the exposure apparatus with inert gas without consuming a large amount of purge gas. FIG. 12 shows a flowchart for explaining the nitrogen purge process disclosed in Japanese Patent Application, Publication No. 6-216000.
However, the proposal in Japanese Patent Application, Publication No. 6-216000, causes a standby time period after the flow rate of the purge gas reduces. This is because the reduced flow rate of the purge gas deteriorates the exhaust performance in the chamber for housing the exposure apparatus's optical system, and causes backflow of the air outside the chamber and rapid increases of oxygen concentration and organic concentration of the material. As a result, the exposure conditions are not met temporarily.
In particular, an F2 laser for a next-generation exposure apparatus has a light absorption factor to oxygen one hundred times as large as that of the ArF laser. In order to obtain the light intensity using the F2 laser equivalent to the ArF laser, both the oxygen concentration and water concentration should be maintained to be below 10 ppm. In addition, since it is anticipated that the F2 laser is more sensitive to blurring due to organic materials than the ArF laser, it is forecasted that a slightly increased amount of oxygen and organic materials affect the exposure performance, and the downtime becomes long.
Accordingly, there is a demand for an inert-gas purge method that inexpensively reduces the impurity concentration in the atmosphere around the optical system in an exposure apparatus, and shortens the downtime. In addition, there are other demands for an exposure apparatus, a device fabrication method using the exposure apparatus, and manufactured devices, in which the exposure apparatus utilizes the above inert-gas purge method to improve the exposure performance and efficiency, and maintains high precision and high throughput.